The Provider-class freighter was a bit of a logical outgrowth from the smaller Zweireiner-class freighter. The smaller design was based on having the larger 20m^3 containers connected to a central spine, but the containers could also be connected to one another for additional structural strength.
So following in on that is the Provider. While the Zwei had a more “Great Lakes” design style, with the command and accomodations forward, freight in the middle, and engines aft, the Provider instead has its engines, command, and most of its accommodation spaces all aft together. Down the middle row of containers is a single structural spar that containers attach to, providing structural support for the attached containers.
One thing that always bugged me with Starfire is that, economically speaking, freighters got more expensive to run on a per hull-size basis as they grew larger, not cheaper. Which is something that’s a little at odds with how bulk freight has always trended (going from galleys to fluyts to east indiamen to steam powered coastal luggers and then to liberty ships and finally diesel fuelled modern freighters), with every new advance making ships cheaper to run, faster, or carrying more cargo. In Starfire it only does one of these (the latter) and the other two are more or less ignored.